It is well known by those in the field of transmission and distribution of electrical energy that the continuity of service during electrical storms is highly dependent on the quantity and quality of the grounds in the vicinity of a lightning strike. Not incidentally, the damage done to cables, transformers and other apparatus is significantly reduced by adequate grounding. Therefore, one of the primary preventive maintenance activities currently by electric utilities is to check and improve as necessary all distribution pole grounds. Every pole in a distribution system should have a ground connection of sufficiently low ohmic value to safely conduct away the current caused by a lightning strike at or near the pole. The magnitude of the task of checking grounds is only appreciated when one considers that a large utility may have as many as 500,000 poles in the distribution system.
The presently accepted way of measuring the resistance of a ground is by the use of a ground resistance tester such as the Biddle Megger.RTM. instruments (Cat. No.'s 250260 and 250241) through the buried grounding conductor (ground rod). Such measurement requires a two or possibly three terminal mesurement to obtain sufficient accuracy. Typically, a single ground rod measurement is made employing the three terminal technique. For reasonable accuracy it is necessary in a typical test, for example, to place a potential electrode at 31 ft. from the rod under test and the current electrode at 50 ft. This is obviously a time consuming task when one considers the time required to dig down as much as 12 inches, break the neutral system grounding wire connection to the rod and remake the connection after the test. If lower accuracy is acceptable, a two terminal measurement may be made. This test nevertheless requires breaking the neutral to ground rod connection and making the measurement between the system neutral and the ground rod. Again the connection must be remade after the test. It is estimated that even the two terminal test could take from 20 to 30 minutes to do, employing a two man crew. At a cost which amounts to $33 to $50 cost per pole. This multiplies to 17 to 25 million dollars for a large system.
From these estimates it is clear that a system is needed to test at least 10 to 20 poles at a time. This necessitates making a ground measurement without breaking the individual grounding wire (neutral) to ground rod connection of all of the poles.